Melatonin priming enhances maize seedling drought tolerance by regulating the antioxidant defense system.

Drought stress (DS) challenges sustainable agriculture production by limiting crop growth and development. The objective of the study was to evaluate the effect of melatonin-priming on enzymatic and non-enzymatic antioxidant defense mechanisms and its relation with leaf ultrastructure and stomatal traits in maize (Zea mays) seedlings under DS (PEG-6000). Drought stress drastically decreased seed germination, plant growth, and leaf chlorophyll content due to excessive reactive oxygen species (ROS) production. Melatonin-priming significantly (P < 0.05) increased seed germination, root length, shoot length, fresh seedling weight, proline content, total soluble protein content, sugar content, chlorophyll content, and stomatal aperture size by 101, 30, 133, 51, 22, 59, 54, 20, and 424%, compared to no priming (NP) under DS, respectively. Similarly, priming improved leaf ultrastructure and reduced the amount of chlorophyll loss and oxidative damage in maize seedlings. Melatonin seed-priming with 500 µM melatonin (M2) greatly increased superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione (GSH), and ascorbate (AsA) activity, by 65, 63, 94, 41, and 55% compared to NP under DS and by 0.26, 8, 33, 42, and 15% under no-stress (NS), respectively. Melatonin-priming also reduced malondialdehyde (MDA) content, electrolyte leakage, hydrogen peroxide (H2O2) content, and superoxide anion (O2‾) content by 26, 31, 31, and 33% compared to NP under DS and by 8, 18, 10, and 11% under NS, respectively. In response to DS, melatonin-priming also stabilized the chloroplast structure, sustained cell expansion, protected cell walls, and greatly improved stomatal traits, including stomatal number, length, and width. Our results suggest that melatonin-priming improves drought tolerance in maize seedlings by alleviating the negative effect of ROS.